1. Field of the Invention
The present invention relates to a spread illuminating apparatus of side-light type used as an illuminating means for a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display (hereinafter, referred to as “LCD”) characterized by its low power consumption, low profile and light-weight has been extensively used as the display mainly for applications of personal computers and cellular phones, and the demand therefor has been increasing. Since a liquid crystal which is a structural element of the LCD does not emit light by itself unlike a light emitting type element such as a CRT, the LCD requires a separate illuminating means for observing an image. In particular, to satisfy great demand for thinning the apparatus and for the apparatus with a lower electric power consumption in recent years, a sheet-like spread illuminating apparatus of side-light type (light conductive plate type) is often used as an illuminating means for irradiating the LCD.
FIG. 5 shows basic components of a spread illuminating apparatus 1. The spread illuminating apparatus 1 mainly comprises a light conductive plate 2 and a bar-like lamp (lamp shaped like a bar) 11, and a bottom side 5 of the light conductive plate 2 is disposed on an observation surface of a reflection type liquid crystal display element (not shown). A light reflection pattern 7 is disposed on a top surface 6 of the light conductive plate 2. The light reflection pattern 7 comprises a plurality of grooves 8 substantially triangular in section and flat portions 9 adjacent to each of the plurality of grooves 8. In the light reflection pattern 7, the pitches of each of the grooves 8 are different at their positions so that brightness is substantially uniform at any position of the light conductive plate 2 irrespective of the distance from the lamp 11. This means that a ratio of a width (an occupied area) of each of the grooves 8 to a width (an occupied area) of each of the flat portions 9 is proportional to a distance from one side surface 3 of the light conductive plate 2. Since the grooves 8 of the light reflection pattern 7 formed on the light conductive plate 2 are minute, they cannot be visually recognized in observing a screen.
The lamp 11 comprises a light conductive bar 13 made of a bar-like transparent material, and a spot-like light source 12 disposed on either of both ends of the light conductive bar 13. A light emitting diode is used as the light source 12, the light conductive bar 13 made of the transparent material is disposed along the side surface 3 of the light conductive plate 2 at a predetermined distance, and the light source 12 is disposed close to one end 14, in FIG. 5, of the light conductive bar 13. An optical path conversion means 15 is formed on the light conductive bar 13. In an example shown in the figure, the optical path conversion means 15 comprises rough portions each substantially triangular in section and is disposed in the longitudinal direction of the light conductive bar 13 according to a predetermined rule.
A longitudinal periphery other than a surface facing the light conductive plate 2 of the light conductive bar 13 is covered by a light reflection member (a reflector) 16 in order to guide the light beam into the light conductive plate 2 at a high efficiency. The light reflection member 16 is formed, by bending a sheet-like film having been evaporated with a metal such as a silver, a sheet-like white film, a sheet-like film having been coated with a white paint, or a mirror-finished plate made of a metal such as an aluminum.
When such a spread illuminating apparatus 1 is incorporated into a reflection type liquid crystal display element (not shown), the light emitted from the bar-like light source 11 enters the light conductive plate 2 through the side surface 3 of the light conductive plate 2, and gradually emitted from the bottom surface 5 of the light conductive plate 2 while repeating reflection and refraction inside thereof and advancing toward a facing surface 10, so that the light illuminates the reflection type liquid crystal display element disposed adjacent to the light conductive plate 2. In addition, since the light reflection pattern 7 is formed on the light conductive plate 2, the amount of the light emitted from the bottom surface 5 can be distributed in a substantially uniform manner.
FIG. 6 shows an example of the conventional spread illuminating apparatus 1 shown in FIG. 5. A light emitting diode of a chip is used as the spot-like light source 12 for a size reduction, and mounted on a flexible printed circuit board (hereinafter, referred to as “FPC”). The light conductive bar 13 is covered by a reinforcing frame 21 made of a metal such as a stainless steel, and also the light source 12 is fixed, with an adhesive resin 22, to a support portion 21a provided on an end of the reinforcing frame.
FIG. 7 shows a cross-sectional view through the line A—A in FIG. 6. Components of the spread illuminating apparatus 1 are integrated and protected by a housing frame 23 when the spread illuminating apparatus is incorporated as the display device into any electronic product such as a cellular phone, thereby the efficiency of the handling in the assembly and the disassembly is improved. The housing frame 23 is a frame-like resin-molded member, and the light conductive bar 13 covered by the reinforcing frame 21 (and also by the light reflection member 16) and the light conductive plate 2 are housed by the housing frame 23. The reflection type liquid crystal display element (not shown) adjacent to the bottom surface 5 of the light conductive plate 2 can be observed from an opening 23a in the housing frame 23 through the light conductive plate 2.
However, the conventional spread illuminating apparatus 1 of the above configuration creates the following problems. First, as shown in FIG. 6, the light source 12 is fixed to a support portion 21a of the reinforcing frame with an adhesive resin 22, and it has been difficult to consistently implement the correct positioning with respect to the light conductive bar 13 because the light source is manually fixed thereto. Second, the fixing work is done by firstly fitting the light conductive bar 13 into the reinforcing frame 21, secondly fitting the light source 12 into a very small space, and finally, adhering the spot-like light source thereto. As a result, great skill is essentially required to obtain a desired assembly accuracy, and the workability is not so good. Further, if the FPC is also fixed at a position deviated from a predetermined position as a result of the light source 12 deviating from its correct position, a short-circuit can occur due to the contact with other metal components.
Third, since the reinforcing frame 21 is made of a sheet metal, and it is generally more difficult to control the accuracy of this reinforcing frame than the resin-molded one. Therefore, the improvement of the assembly accuracy has its limits because the light source 12 is fixed to the support portion 21a of the reinforcing frame which is apt to be dislocated. If the light source 12 is dislocated with respect to the light conductive bar 13, the brightness of the spread illuminating apparatus 1 is directly affected. Therefore, the consistency of the assembly accuracy is essential for improving the quality of the spread illuminating apparatus 1.
Finally, the housing frame 23 has to have a certain level of thickness enough to secure the necessary strength. As a result, in incorporating the housing frame 23 into any electronic product, the light conductive plate 2 and the reflection type liquid crystal display element (not shown) are positioned toward the recess by the thickness of the housing frame 23 from a front face of any electronic product (a front face of a display unit). Here, it is scientifically proved that the human's sense is keen enough to have a feeling of a remoteness even by the thickness of the housing frame 23, whereby a conventional structure in FIG. 7 cannot provide a satisfactory visibility as the display device.
Thus, the light conductive plate 2 and the reflection type liquid crystal display element (not shown) are adapted to be positioned as close to the front face of the concerned electronic product as possible, by making the light reflection member 16 in contact with the housing frame 23 and also by replacing a reinforcing frame U-shaped in section with a flat reinforcing plate 24 shown in FIG. 8. However, a problem occurs in that it is more difficult to secure the strength of the structure using the reinforcing plate 24 than that of a structure using the reinforcing frame 21 U-shaped in section.